This invention relates to the measurement of the density or solids content of sludge such as sewage sludge, low solids slurry or like material, containing gas bubbles.
The treatment of sewage sludge or like slurries generally includes a de-watering process. To assist this process it is normal practice to add a material to the sludge to aid the separation of solids from the liquid, such additives being, for example, synthetic flocculants, polyelectrolytes and polymers. The dosage required for these additives is normally directly related to the solids content of the sludge. It follows, therefore, that for optimum treatment of the sludge that an accurate measurement of the solids content of the sludge is needed.
Known methods for finding the solids content of sludge comprise measuring the sludge mass flow, for example from a sludge bulk storage vessel, and measuring the liquid carrier mass flow from which the mass flow of the solids can be calculated as follows:
Solids mass flow=Sludge mass flowxe2x88x92Liquid carrier mass flow.
A problem with this kind of measurement is that due, for example, to the sludge digestion process or natural fermentation many types of sludge contain dissolved gas, entrapped gas bubbles or both. The presence of this gas can seriously affect the measurement of the sludge density or solids content. Detectors based on ultrasound or microwaves will xe2x80x9cseexe2x80x9d gas bubbles as high solids particles whereas for those gauges which measure sludge density on radiometric or coriolis principles the presence of gas bubbles has the effect of giving a reduced reading for the density. Moreover the gas content of the sludge is very likely to be variable so that it is not possible to provide for some reliable standard compensation for gas content in the solids content calculation.
In order to deal with this problem degassers have been fitted to the main sludge line. While these help to some extent it has been found that in practice accurate on line measurement of sludge density is not possible. It has therefore been proposed to provide a by pass system having a solids/density gauge through which a sample of sludge from the main sludge feed line is passed under controlled conditions. After the measurement has been taken the sample may be returned to the main sludge line, returned to the bulk sludge storage vessel or merely drained away. In some cases the sample is subjected to increased pressure in order to try to reduce the effect on the solids/density measurement of gas bubbles in the sludge. However there are some sludges whose gas content is too high and/or too variable for these proposals to yield reliable results.
The present invention has been made in order to deal with these difficulties.
According to the invention there is provided a process in which a sludge treatment stage is controlled in response to the density or solids content of the sludge, comprising
flowing sludge along a flow line towards the treatment stage,
taking a sample from the flow line,
degassing said sample to produce a degassed sample,
measuring the density or solids content of the degassed sample, and
utilising the measured density or solids content of the degassed sample for controlling the treatment stage.
Degassing of the sample may be conducted by continuously cycling the sample as a batch through degassing apparatus. During such cycling degassing comprises feeding the sample into a first tank and flowing the sample down an inclined plane into a second tank. The degassed sample may be made by continuing the degassing step of the process until a substantially constant value for solids content is measured. Alternatively the degassed sample may be made by continuing the degassing step for a predetermined time.
In one embodiment of the invention the sample is taken from the flow line through a feedpipe and admitted to the main chamber of a twin-chambered tank through a valve. As a result the level of sludge in the main chamber rises until it reaches a sensor having a switch which terminates the sludge inflow into the main chamber by closing said valve. Sludge is withdrawn from the main chamber through a second valve for example by suction supplied by a pump. The density or solids content of the sludge is measured by one or more on-line density or solids content measuring devices, a positive pressure being maintained on the sludge by said pump during said measurement. The said pressure is preferably held at a constant level during said measurement for example by means of a back pressure valve. After said measurement the sludge is conducted to the other chamber of said twin-chambered tank from whence it flows over a weir and down an inclined plane into the said main chamber. Sludge is either recirculated through the system or removed from the main chamber through valve means and transferred to a storage vessel or returned back into the flow line. If desired the twin chambered tank can be purged with fresh sludge from the flow line before degassing is carried out.
The invention can be applied particularly to sludge treatment which comprises treatment with flocculent and/or coagulant. Treatment can comprise dewatering to produce a cake.
The invention also provides apparatus for degassing sludge, comprising
means for extracting sludge from a flow line, and
a degassing circuit, comprising
a degassing tank,
means for withdrawing sludge from the degassing tank,
one or more density or solids content measuring devices,
means for transferring sludge from the degassing tank to said one or more density or solids content measuring devices,
means for redirecting sludge back to said degassing tank from said one or more density or solids content measuring devices, and
means to divert degassed sludge from said degassing tank.
Sludge diverted from said degassing tank, for example through an outlet valve, may be returned to the flow line or may be transferred to bulk storage.
In a preferred embodiment of the apparatus of the invention a feedpipe is provided for taking sludge from the flow line, there being a valve for controlling the flow of sludge through the feedpipe. A twin chambered degassing tank receives sludge from the feedpipe. The degassing tank has first and second chambers separated by a weir and an inclined plane and a sensor for detecting the level of sludge in the degassing tank and for controlling the admission of fresh sludge to the degassing tank. An outlet valve is provided in the degassing tank for transferring sludge to the one or more measuring devices and a pump acting against a back pressure valve provides for a positive pressure to be imposed on the sludge while it is subject to density or solids content measurement.
In this specification the term xe2x80x9cdegassedxe2x80x9d does not mean necessarily that there is total removal of gas from the sludge but that the amount of gas in the sludge has been reduced to a level where stable density or solids content measurements are possible. References to xe2x80x9cdegassingxe2x80x9d of sludge are to be understood in the same way.